1. Field of the Invention
The present invention is directed to apparatus for measuring leakage of fluids from a closed container and more particularly to a pressure indicator utilizing a helical bourdon tube sensing element for monitoring the pressure in aircraft tires and the like.
2. Description of the Prior Art
It is well known in the aircraft industry, that the aircraft tire pressure is extremely important and must be maintained within a limited range. For example, the tire pressure for a DC-10 should be between 170 and 185 psi at 68.degree. F. and atmospheric pressure. As can be readily appreciated, a tire in a modern jet aircraft is subject to extreme variations in ambient pressure and temperature during normal usage. Additionally, the tire itself is subject to extreme force loadings during a landing and an underinflated tire can cause a blow out and even a serious crash.
The high cost of capital equipment in the aircraft industry requires the maximum utilization of a plane. This utilization, however, must be balanced against safety requirements which frequently provide opposing criteria to be met. Thus, while it is desirable to permit ground crews to monitor various equipment and components of an aircraft prior to a takeoff, it is also highly desirable to simplify this procedure and to permit it to be conducted at the lowest possible human fail rate and with the most expedient and simplest visual checks. Besides aircraft tires, fire extinguisher containers on a modern aircraft should frequently be checked to be sure that they have not been rendered useless as a result of a leak. These fire extinguisher units in aircraft assemblies can experience relatively high pressures of 3,000 psia or greater and sudden temperature changes of several hundred degrees Fahrenheit which makes it difficult to accurately determine the condition of the unit.
One particular leakage indicator for fire extinguishers is shown in U.S. Pat. No. 3,735,376 issued on May 22, 1973 and assigned to the assignee of the present application. The device of U.S. Pat. No. 3,735,376 provides a temperature compensated alarm mechanism for a fire extinguisher suitable for use on aircraft or other vehicles. The mechanism included, in addition to the main pressure container, a miniaturized container which was inserted into the main chamber and was effectively separated by a diaphragm between the two containers. The diaphragm was responsive to a predetermined drop in pressure of the main container to produce a signal indicative that the fire extinguisher was leaking. Since both the miniature container and the main container were filled with fluids having similar pressure temperature responses, the device was essentially temperature independent.
Another temperature compensated pressure indicator is disclosed in U.S. Pat. No. 3,946,175 issued Mar. 23, 1976, and assigned to the assignee of the present application. In this apparatus, a bellows member formed of welded metal rings was sealed in the indicator housing between the primary and secondary pressure chambers and was capable of indicating any differential pressure.
Patents of general interest are U.S. Pat. No. 3,905,237, U.S. Pat. No. 3,163,046 and U.S. Pat. No. 3,990,309.
There is still a demand in the prior art to provide a relatively simple and economical pressure gauge assembly that is capable of providing a visual indication of leaks in a monitored pressure chamber such as fire extinguisher assemblies. Additionally, the monitoring of pressure of aircraft tires in a quick and efficient manner is a requirement that has not to date been satisfied by the prior art.